Computer numerical control (CNC) machine tools such as routers used in the woodworking, plastics and nonferrous metal industries, typically consist of a base unit, a stationary or movable workpiece support table mounted on a base unit, a stationery or movable gantry mounted on or adjacent the base unit, and a toolhead assembly mounted on a transversely disposed bridge member of the gantry. Either the table or the gantry is displaceable relative to the base unit longitudinally or along an x-axis, the toolhead assembly is displaceable transversely or along a y-axis and the toolhead is displaceable vertically or along a z-axis. Each is displaced along their respective axes by feedscrews driven by servomotors. The motions of the various components of the machine are controlled by a controller that operates the various servomotors of the machine according to instructions of a program inputted into the controller.
Workpieces to be machined are positioned on the table in predetermined locations, and are held down by various means including clamps and vacuum systems. Vacuum systems may consist of conventional systems that provide high vacuum, suitable for large production runs, and universal systems that are more suitable for short production runs. A conventional vacuum system generally includes a vacuum port provided in the worktable, connected to a vacuum pump and a vacuum fixture positioned on the workpiece table about the vacuum port on which the workpiece is positioned. The fixture is provided with a peripheral rubber seal engaged by the workpiece seated thereon, which permits the evacuation of air between the fixture and the workpiece to hold the workpiece in place. A universal vacuum system also known as a high flow system, generally includes a table having a lower rigid plate, an arrangement of spacers attached thereto in a grid pattern, a perimeter wall, an upper spoilboard formed of a porous material such as particleboard supported on the spacers and perimeter wall, closing the spacer grid area to form a plenum, and vacuum pump operatively connected to the plenum. As a vacuum is applied to the plenum, air is drawn through the porous upper board material, producing a low-pressure zone at the surface, which functions to hold a workpiece positioned thereon.
Each of the above-described systems has certain disadvantages. In the case of the conventional system, a discrete NC program must be developed to facilitate the routing of the perimeter gasket groove for each individual workpiece to be mounted. Each fixture then becomes dedicated to the workpieces for which it was adapted to hold.
Multiple fixtures must be prepared to facilitate the fastening of a variety of different workpieces. Consequently, the maintenance and storage of a sizable quantity of these fixtures is often required, which in turn, consumes a considerable amount of time and resources.
The alternate, high-flow method was developed in an effort to reduce the cost of preparing, storing, and changing spoil boards. However, because this method requires air to flow at a very high volume, it necessitates the use of an expensive, high volume pump that consumes a considerable amount of electrical energy.
Because air is drawn through the entire surface of the worktable, even while the work function is being performed, most of the energy required to operate the high-flow system is wasted. In addition to expensive vacuum pumps, both systems described in the foregoing paragraphs require an extensive array of peripheral items such as pipes, hoses, and valves.
In certain operations, such as nested based panel processing, the high-flow system is the only workable alternative. The expense involved in such a system adds considerably to the cost of a CNC machine installation. In some cases, the cost of a high-flow system can amount to as much as 30 percent of the total cost of a machine installation. CNC machine ownership thus becomes cost-prohibitive for many small business owners, who would otherwise stand to derive considerable benefit from CNC machining technology.
An object of the present invention is to provide a cost-effective vacuum operated hold-down system. A second object of the present invention is to provide a vacuum hold-down system that is an integral component of a CNC machining system. A third object of the present invention is to provide a modified cutting technique that allows the processing of smaller panels utilizing the high-flow system. Further objects of the present invention will become apparent, based on the following detailed description.
These and other objects are overcome in a system that utilizes a relatively low-cost centrifugal type impeller, coupled to an a-c induction motor. It is relatively light in weight, which facilitates the mounting of the entire system directly to the underside of the moving table of a CNC machine. By building the system into the machine, it is possible to place the inlet of the blower directly under the worktable vacuum port, thus eliminating all external piping. Since there is no need for piping within system, static pressure losses between the vacuum source and the worktable plenum are virtually non-existent. The system is simple and inexpensive.
The aforementioned technique produces less vacuum than the high-vacuum and high-flow systems currently in use. This lower level of vacuum results in a reduced force per square inch of part surface holding the part to the worktable. In this case, the total force holding smaller parts may be insufficient to counteract normal cutting forces and the part may move during the cutting process. This deficiency is overcome by the use of a modified cutting technique, which is automatically initiated in areas where the workpiece falls below a predetermined size. The technique comprises a multi-stage cutting cycle, whereby a first cycle leaves material in the bottom of the cut path on parts that fall below a predetermined size. The material remaining at the bottom of the cut path secures the small part to the adjoining parts during the cutting process. The remaining material is cut away in a second pass. Because the thickness of the remaining material is in the order of 10 thousandths of an inch, the cutting force required to remove the material is extremely low and will not move the workpiece, despite the relative low hold-down force. If however, the size of the part falls below yet a lower predetermined value, the speed of the cutting cycle will likewise be reduced by an amount sufficient for preventing movement during the cut cycle.